My invention relates generally to an apparatus for the transfer (reading and/or writing) of data with disklike record media such as magnetic disks notably including those of the "fixed" or hard disk variety. More particularly, my invention pertains to a positional control system for a data transducer or head in such an apparatus in accessing the transducer to any of a multiplicity of concentric data tracks, or to any radial position on a single multiturn spiral track, on the rotation disk.
The majority of hard disk drives commercially available today employ some form of rotary or linear actuator such as a voice coil motor under closed loop electronic servo control for positioning the transducer in centerline alignment with any of the data tracks on the disk. For each such track seeking operation the transducer positioning system generates a seek speed signal for causing transducer movement at an optimum speed depending upon the distance from the current transducer position to the destination track. Also, during the track seeking operation, a signal is generated which represents the actual traveling speed of the transducer. Both the seek speed signal and the actual transducer speed signal are directed into a differential amplifier. The resulting signal representative of the difference between the desired and actual transducer speeds is then applied to a motor drive amplifier via a low pass filter (LPF).
The LFP serves to make transducer movement smoother by removing unnecessary higher frequency components from the difference signal. This advantage has so far been nearly offset, however, by the phase lag introduced by the LPF into the closed loop servo control system. The phase lag has resulted in the tendency of the transducer to travel back and forth past the destination track.
It might be contemplated to alleviate this inconvenience by making the cutoff frequency of the LPF higher. This solution is unsatisfactory for two primary reasons. One is that the transducer would be subjected to greater variations in speed while traveling at high speed, resulting in the production of greater audible noise than heretofore. The other is that the feedback control system would become unstable in operation.